Such systems are principally known in the art. It is especially known that the gain μ of a scintillation detector system, comprising a scintillator, a photocathode and a photomultiplier tube (PMT) together with an evaluation system is subject to a change in gain over time. The gain change of the overall system is substantially effected by the gain change of the PMT. That gain change is due to environmental changes, i.e. a modification in temperature over time or other environmental factors.
In order to stabilize the gain of the PMT, it is known in the art to conduct several measurements over time and to compare the results. An initial or reference measurement may take place at beginning of the first measurement of nuclear radiation, for example using a calibration source with well-known energies of the emitted gamma radiation. The light signals, produced by the gamma radiation in the scintillator crystal, are proportional to the energy deposed in that crystal. The light signals do then hit the photocathode, that photocathode emitting electrons, which are collected by a PMT. A PMT consists of a series of dynodes and a final anode. The usually very few photoelectrons from the photocathode are accelerated towards the first dynode where they produce a multitude of electrons, being emitted from that first dynode. Those electrons are then accelerated to the next dynode, where their number is again multiplied by the same factor, those electrons being led to the next dynode and so on, until they finally reach the anode of the PMT, where a current signal is measured, being proportional to the charge of the multitude of electrons. That charge is proportional to the amount of light, generated in the scintillator and therefore proportional to the energy deposed by the gamma radiation in the scintillator.
The resulting charge signal is then further processed and usually stored in a multichannel analyzer (MCA), each channel of that MCA corresponding to a specific radiation energy, deposed in the scintillator crystal. An accumulation of such energy signals results in an energy spectrum, each line in that spectrum corresponding to a specific energy deposed in the detector system.
For most applications it is of interest to obtain the best resolution in energy a system allows. One of the problems, leading to a decrease in energy resolution is the gain shift, which is to be avoided therefore.
In order to do so, it is known to measure the gain at different times, using gamma radiation with known energy. This gamma radiation with known energy may be emitted by a calibration source, or may be another known energy, being present in the spectrum to be measured anyway. The gain of the two measurements at different times is compared and the signals are corrected by the difference, therefore multiplying all signals by a so-called gain correction factor, thereby stabilizing the overall system.
It is also known to use artificial light pulses instead of light pulses, generated by the scintillation crystal following the absorption of radiation energy. Such an artificial light source may be an LED.
The disadvantage of all these known systems is that one has to know either a specific—constant—line (energy) in the spectrum to be measured or to use a calibration source, thereby interrupting the measurement from time to time. In addition, especially at high count rates, it may be difficult to obtain a stabilization spectrum at all.
The aim of the present invention is therefore to avoid the above-mentioned disadvantages and to provide a self-stabilizing scintillation detector system without the need of identifying specific lines in the output spectrum, identified as calibration sources, and to correct the gain on the basis of the shift of those lines.